How important is groundwater availability and stream perenniality to riparian and floodplain tree growth?

Abstract

Riparian vegetation is important for stream functioning and as a major landscape feature. For many riparian plants, shallow groundwater is an important source of water, particularly in areas where rainfall is low, either annually or seasonally, and when extended dry conditions prevail for all or part of the year. The nature of tree water relationships is highly complex. Therefore, we used multiple lines of evidence to determine the water sources used by the dominant tree species Eucalyptus camaldulensis (river red gum), growing in riparian and floodplain areas with varying depth to groundwater and stream perenniality. Dendrometer bands were used to measure diel, seasonal, and annual patterns of tree water use and growth. Water stable isotopes (δ2H and δ18O) in plant xylem, soil water, and groundwater were measured to determine spatial and temporal patterns in plant water source use. Our results indicated riparian trees located on relatively shallow groundwater had greater growth rates, larger diel responses in stem diameter, and were less reactive to extended dry periods, than trees in areas of deep groundwater. These results were supported by isotope analysis that suggested all trees used groundwater when soil water stores were depleted at the end of the dry season, and this was most pronounced for trees with shallow groundwater. Trees may experience more frequent periods of water deficit stress and undergo reduced productivity in scenarios where water table accessibility is reduced, such as drawdown from groundwater pumping activities or periods of reduced rainfall recharge. The ability of trees to adapt to changing groundwater conditions may depend on the speed of change, the local hydrologic and soil conditions as well as the species involved. Our results suggest that E. camaldulesis growing at our study site is capable of utilizing groundwater even to depths >10 m, and stream perenniality is likely to be a useful indicator of riparian tree use of groundwater.

Ideas and perspectives: Tracing terrestrial ecosystem water fluxes using hydrogen and oxygen stable isotopes – challenges and opportunities from an interdisciplinary perspective

“…Dudley et al (2018) saw little evidence to suggest that landscape position altered groundwater uptake by shrubs. Pettit and Froend (2018) showed that riparian trees located on relatively shallow groundwater had greater growth rates, larger diel responses in stem diameter and were less reactive to extended dry periods than trees located in areas of deep groundwater. Sprenger et al (2018c) found that soil water beneath conifer trees was more fractionated than beneath heather shrubs or red oak trees and that sampling locations closer to streams had a more depleted isotopic composition than hillslope sites, revealing increased subsurface mixing towards the saturated zone and a preferential recharge of winter precipitation.…”

Ecohydrology of urban trees under passive and active irrigation in a semiarid city

“…In the case of summer 2017, PI greenness responded to precipitation pulses similar to vegetation in natural desert ecosystems [48]. Conversely, AI greenness mimicked the behavior of trees in water-abundant riparian systems, where precipitation elicits little physiological response to trees with perennial groundwater availability [34,38]. In this vein, it is possible that rain events have diminished influence over urban mesquite tree greenness when active irrigation is already maintaining an elevated soil moisture profile.…”

“…In addition to depth, soil moisture temporal dynamics play key roles in desert functioning at the plant and ecosystem scale [32–34]. For example, the water-limited southwest United States experiences peak primary productivity during summer rains, but an extended period of abundant soil moisture in the spring—usually following a wet winter—can increase carbon uptake earlier in the year [35–37].…”

Identification of Source‐Water Oxygen Isotopes in Trees Toolkit (ISO‐Tool) for Deciphering Historical Water Use by Forest Trees

“…Studies which provide detailed ecohydrological information are typically based on comparisons between the contemporaneous measurements of tree xylem waters with those of local water sources (e.g., Plamboeck et al, 1999;Sánchez-Pérez et al, 2008). While direct analysis of xylem δ 18 O circumvents the leaf fractionation and exchange mechanisms which mask the source water δ 18 O information stored in δ 18 O cell (McCarroll & Loader, 2004), xylem isotope studies are limited by the temporal domain of field work (typically 2-3 years), restricting the development of broader conclusions about ecohydrological interactions (Pettit & Froend, 2018). Long-term, seasonal reconstructions of δ 18 O in tree water source could yield potentially powerful new information about water availability to trees, and thus hydrological processes in the critical zone, while also providing historical context for realtime investigations.…”